LAPIS Semiconductor ML610421, User Manual, Page: 320 / 437

ML610Q421/ML610Q422/ML610421 User’s Manual
Chapter 24 RC Oscillation Type A/D Converter
24 – 16
To convert from an RT0 value to a digital value, the ratio is used between a) the oscillation frequency by the thermistor
connected to the RT0 pin and the capacitor connected to the CS0 pin and b) the oscillation frequency by the reference
resistor (which ideally should have no temperature characteristics) connected to the RS0 pin and the capacitor
connected to the CS0 pin. This is for making the conditions other than resistance equal to eliminate the error factor in
oscillation characteristics.
As shown in Figures 24-9 and 24-11, the RT0 value depends on temperature T and the RS0 value is assumed to be
constant regardless of temperature T. It is ideal if the characteristics of the oscillation frequency fOSC to temperature
T using these resistances will be like the solid lines in Figures 24-12 and 24-13; however, in reality, it would appear that
they will be like the dotted lines due to error factors such as IC temperature characteristics.
Since the condition of f
RCCLK
(RT0) and that of f
RCCLK
(RS0) are the same except for the resistances, the error ratios are
almost the same; therefore, errors can almost be eliminated by using the ratio between f
RCCLK
(RT0) and f
RCCLK
(RS0).
The ratio between f
RCCLK
(RT0) and f
RCCLK
(RS0) is equivalent to the above-mentioned A/D conversion value nT0 that
should ideally depend only on RT0.
R
ef
er
e
nc
e r
es
is
tanc
e
RS
0
Temperature T
f
RCCLK
(RT0)
Temperature T
Ideal
Includes errors due to factors other
than RT0
f
RCLK
(RS0) =
k
RCCLK
·(CS0+CVR)·RT0
1
Figure 24-11 Temperature Characteristics of
Reference Resistor
Temperature T
Includes errors due to factors
other than RT0
f
RCCLK
(RS0) =
k
RCCLK
·(CS0+CVR)·RS0
1
Ideal
f
RCLK
(RS0)
Figure 24-13 Oscillation Characteristics of Reference Resistor
Figure 24-12 Oscillation Characteristics
of Thermistor